Flow through venous cannulae.

Fluid flow through modern cannulae is not simple and cannot be expressed in classical terms. Progressively increasing the length of a cannula diminishes flow predictably but altering the bore of the cannula does not give a fourth power improvement in flow but rather a linear one. A reasonable working value for the relative viscosity of ACD blood in infusions would seem to be 2-6 below about 125 ml/minute flow (saline). The decline in relative viscosity above this may be caused by turbulence in the saline flow.